Process of forming electrostatic printing paper from ionic fibrous cellulosic derivatives and paper thereof



United States Patent 3 420 734 PROCESS OF FORMING ELECTROSTATIC PRINT-ING PAPER FROM IONIC FIBROUS CELLULOSIC DERIVATIVES AND PAPER THEREOFNils T. Anderson, Vancouver, Wash., and William M.

Hearon, San Rafael, Calif., assignors to Crown Zellerbach Corporation,San Francisco, Calif., a corporation of Nevada No Drawing. Filed Apr. 2,1965, Ser. No. 445,271 U.S. Cl. 162135 9 Claims Int. Cl. G03f /02ABSTRACT OF THE DISCLOSURE In electrostatic printing, a method ofdissipating electrical charges on the back of a dielectric film providedfor receiving an image-defining electrostatic charge, where the chargeson the back of the film are conducted through a cellulosic materialhaving electrical conductive properties occurring by reason of ionicdisassociation taking place in the cellulosic material. An electrostaticprinting paper for practicing the method including a cellulosic basesheet having a fibrous structure composed of an ionic fibrous cellulosicderivative, which is the product formed by substituting ionizable groupsin the reactive groups of the anhydroglucose units that form cellulose,the printing paper including a dielectric film extending over at leastone face of the base sheet.

This invention relates to the manufacture of ionic cellulosicderivatives, products prepared from such derivatives, and processes formaking such products.

The cellulosic derivatives of the invention have been found to beparticularly suited for the manufacture of electrostatic printingproducts, where they may be incorporated in the means supporting aninsulating or dielectric film in such products, with the derivativesthen, by reason of their conductive properties, imparting neededelectrical conductivity to the products.

Electrographic printing, as the term commonly is used today, refers toan electrostatic printing process, wherein an image-definingelectrostatic charge is laid directly down upon an insulating ordielectric surface, using such electronic devices as a cathode ray tube,a pin matrix and pulsing corona discharge, or by means of a direct imagetransfer from one surface to another. A copy product such as a copysheet used in making electrographic prints requires a layer presenting adielectric or insulating surface for the reception of the image-definingelectrostatic charge, which layer may take the form of a thin film ofdielectric material. The copy product further requires support for thislayer which is electrically conductive, whereby any stray electricalcharges in the means supporting the film may be dissipated to enable theproduction of the clearest possible prints. In the usual copy product,this electrically conductive support for the dielectric film is providedby including in the copy product an electrically conductive web, such asa conductive fabric web, or a conductive paper web, which has thedielectric film applied over at least one face thereof.

While relating generally to the manufacture of ionic cellulosicmaterials, in a more specific sense the invention concerns novelelectrically conductive webs produceable from the material, novelmethods of producing such webs, improvements in electrostatic printingproducts featuring as a conductive supporting base therein webscontaining ionic cellulosic derivatives, and novel methods formanufacturing such electrostatic printing products.

One general object of this invention is to provide an improvedelectrically conductive web, such as a paper ice or fabric web, wherethe conductive properties of the web are derived at least in part fromthe inclusion in the web, distributed therein or as a coating, or in anyother manner, of ionic cellulosic derivatives having electricallyconductive properties. An object related to the above is the provisionof a printing product for electrostatic printing, that includes adielectric film for receiving an imagedefining electrostatic charge,where the supporting base for this film is provided by such a web, andthe web imparts the electrical conductivity needed in the supportingbase.

The usual procedures which have been relied upon to provide necessaryelectrical conductivity in a Web emloyed in electrostatic printingproducts have comprised either coating or impregnating the web with ahumectant, coating or impregnating the web with an ionizable inorganicor organic compound, coating or impregnating the web with metallic ororganic electroconductors, or impregnating the web with apolyelectrolyte. With all of these procedures, additives to the web areresponsible for electrical conductivity, and as a consequence, inmanufacturing the web the additional step of incorporating the additivewith the Web at some stage in its manufacture is required, adisadvantageous feature. A further problem experienced with prior-knownpractices relates to obtaining uniformity in the electrical conductanceof a web. In electrostatic printing products optimum imagecharacteristics are favored by uniform conductivity in the web whichforms the supporting base, but in practice such uniformity, whenadditives are employed to obtain conductance, has been difiicult toobtain. Surface imperfections, variations in web thickness, variationsin absorbency characteristics, and variations in the rate of applicationare some of the factors which render it hard to produce a consistentlyuniform product. Therefore, according to a preferred embodiment of theinvention, the ionic cellulosic derivatives contemplated areincorporated into the web as part of the fibrous structure of the web,and by such a procedure the above-indicated ditficulties anddisadvantages are eliminated or reduced materially.

Accordingly, another object of the invention is to provide an improvedelectrically conductive Web, where the conductance in the web is theresult of properties of the fibrous material forming the structure ofthe web, rather than the result of additives applied to the webstructure or coated thereon.

More specifically, an object is to provide a web as described, whereinionic fibrous cellulosic derivatives form at least a portion of thefibrous structure of the web, which derivatives, by reason of theirionic nature and through disassociation, render the web electricallyconductive.

Yet another object is to provide a novel electrostatic printing sheet,featuring a dielectric or insulating film and as a support and providinga conductive base therefor a base web with ionic fibrous cellulosicderivatives forming at least a part of the fibrous structure of the web.

It has been discovered according to this invention that a number ofdifferent types of cellulosic derivatives with ionic characteristics maybe employed as the fibrous material, including both cationic and anionicderivatives. Representative of the former class of derivatives are suchmaterials which ionize in the presence of water and release positivelycharged cations as cellulose phosphates (formed by esterifying thehydroxyl groups in the anhydroglucose units forming cellulose), oxidizedcellulose (where carboxyl groups replace the primary hydroxyl groups ofthe anhydroglucose units), sulfoethyl cellulose, and low substitutedcarboxymethyl cellulose. Illustrative of the latter group of derivativesare such materials as aminoethyl cellulose, diethylaminoethyl cellulose,and guanidoethyl cellulose, which materials, in the presence of Water,ionize and release negatively charged anions. When such materials areincorporated in a web as part of the fibrous structure of the web, it isionic disassociation occurring in the polymer forming the cellulosicfiber in the web which renders the web electrically conductive, asopposed to disassociation of additive substances distributed incellulosic fibers.

With electrically conductive webs perpared from cellulosic derivativesas contemplated herein, uniform electrical conductance results, so thatwhen the webs are coated with dielectric films to make electrostaticprinting products, they provide a uniformly electrically conductive basein the final products. Such a uniformly conductive base has resulted ingood prints being obtained through electrograp hic printing methods withsurface resistivities ranging up to 1.5)( ohms per square (even up tol.0 10 ohms per square in some applications). Heretofore, and withpresently known electrographic printing processes, it has generally beenconsidered that satisfactory image resolution was only obtainable with aresistivity in the base sheet of a printing paper of not more than IX 10ohms per square. Thus, a further feature and object of the invention isto provide an improved electrostatic printing product, morespecifically, a printing paper, which, because of superior homogeneityin the electrical conductance in a base web therein makes possible theproduction of satisfactory image resolution with higher surfaceresistivities in the base web than heretofore believed practicable.

According to a specific and preferred embodiment of the invention, acellulosic derivative comprising phosphorylated cellulose is selected asthe ionic fibrous cellulosic derivative forming the fibrous structure inthe web, and the derivative itself is prepared in a unique mannerwhereby a high degree of phosphorylation is produceable in thederivative. According to the invention, a phosphorylated cellulosicderivative may be prepared containing up to about 18% by weight combinedphosphorus. An electrostatic printing product such as a printing paperhaving such a phosphorylated cellulose forming the fibrous structure inthe base sheet of the paper, containing from to 20% combined phosphorus,has been found to be particularly well suited for producingelectrostatic prints at relatively low humidity conditions. A copy sheetproducing satisfactory prints at a relative humidity of 20% or more issatisfactory for many uses. With printing papers having a base sheetmade of a phosphorylated cellulose containing combined phosphorus in therange indicated, satisfactory image resolution may be obtained at a 15%relative humidity, which is, of course, a desirable factor as it reducesthe extent of control required over humidity conditions.

A further feature and object of the invention is to provide an improvedprocess for making ionic fibrous cellulosic derivatives from cellulose,which makes possible the manufacture of derivatives containing a highproportion of combined phosphorus, the process being characterized bycontrol of temperature conditions and the use of a limited amount ofwater, whereby a high degree of phosphorylation of the cellulose isobtained without degradation of the cellulose.

Various other features and advantages of the invention will becomeapparent from reading the following description, such descriptionincluding certain specific examples, inserted primarily for purposes ofillustrating the invention, and not being intended as indicating theentire range of conditions and procedures under which the invention maybe carried out.

As indicated above, this invention contemplates the manufacture of a webwhich may be used as a base sheet in an electrostatic printing producthaving ionic fibrous cellulosic derivatives forming at least a portionof the fibrous structure of the web. The ionic forms may be cationic oranionic. With the cationic derivatives, upon ionization in the persenceof water, positively charged cations are released, and the choice ofcation may be varied depending upon the properties desired in the web.The particular cation selected for a specific application depends tosome extent on the level of surface resistivity desired. It is wellknown that the hydrogen cation possesses the greatest equivalentconductance, and that among the group of monovalent cations equivalentionic conductance increases in the order: lithium, sodium, silver,ammonium, potassium, hydrogen. Providing the degree of ionization is notchanged by the choice of cation, the l0lWSt surface resistivity isexpected from a paper made from fibrous cationic cellulosic derivativesincorporating the hydorgen ion. In most cases the degree of ionizationis a function of the cationic form, so this also should be considered inthe selection of the cation. Other considerations also are importantsince it has been observed that when, for example, an aluminum ion isincorporated with a cellulose phosphate, a somewhat fluffy sheet thatmay be undesirable in some instances results, whereas the use of theammonium ion results in a dense, more acceptable sheet for printingpurposes.

With anionic cellulosic derivatives which ionize to release negativelycharged anions (aminoethyl cellulose, diethylaminoethyl cellulose, etc.)again, the particular ion incorporated in the derivative may be selectedfrom the group of mono-, di-, or polyvalent anions such as chloride,bromide, iodide, nitrate, hydroxyl, acetate, sulfate and the like, withthe particular selection made depending upon the desired surfaceresistivity and the specific type of paper properties desired. It iswell known that the equivalent ionic conductance for anions increases inthe order: acetate, nitrate, chloride, hydroxyl; and providing thedegree of ionization is independent of the anion, the lowest surfaceresistivity is expected from paper made with fibrous anionic cellulosederivatives incorporating the hydroxyl ion.

The surface resistivity of paper made from fibrous ionic cellulosicderivatives is approximately inversely related to the number ofionizable groups in the derivative. For example, it has been observedthat the surface resistivity of webs made from cellulose ammoniumphosphate decreases by about one-half when the degree of substitution ofphosphate groups in the derivative is doubled. Using present dayprinting processes, and in the process of making electrostatic printsfrom a copy sheet comprising a dielectric film supported on anelectrically conductive paper base sheet, as already indicated, aresistivity of not more than 1X10 ohms per square usually is requiredfor satisfactory image resolution to be obtained. This range may beextended upwardly to about 1.5 l0 ohms per square with paper or otherwebs made of fibrous ionic cellulosic derivatives because of the uniformelectrical conductance produceable therein.

It has been found that any cellulose-containing materials may beutilized in preparing the ionic cellulosic derivatives contemplated inthis invention. Illustrative of such materials, but not being limitedthereto, are papermaking pulps, ground wood or cotton linters, rayon andthe like.

The following example illustrates the preparation of an ionic fibrouscellulosic derivative from cellulose, through phosphorylating thecellulose, using as a reaction medium a mixture of concentratedphosphoric acid and a weak organic nitrogenous base.

Example I An air dry bleached sulfite pulp was immersed for 5 minutes ina solution consisting of 68 parts urea and 32 parts orthophosphoric acid(containing by weight acid and the remainder water) with such solutionmaintained at C. As will be noted, the impregnating solution containedonly a small amount of water. The acid and urea solution had a meltingpoint of about 85 C. and 110 C. temperature at which it was maintainedwas well below that temperature where foaming occurs in the solutionwith loss of chemical (with such acid and urea solution, foaming becomesa factor at about 125 C.). After such'immersion, the pulp was removedfrom the solution and pressed to reduce the amount of solution retainedin the fibrous cellulosi-c mass. The fibrous mass can retain up to about10 times its weight on an air dry basis of the solution, and withrelatively heavy pressures, this retention can be reduced to where theweight of retained solution about equals the weight of the air drymaterial. In the example, after pressing, retained solution equaledabout 3 times the weight of the air dry material.

The cellulosic mass with retained solution was then cured, to bringabout esterification of the hydroxyl groups in the anhydroglucose unitsof the polymer forming the cellulose, by placing it in a convection ovenat 180 C. Different curing times, ranging from 3 to 15 minutes, wereemployed. After curing, the cellulosic material was washed until thewashing gave no test for phosphate. Paper hand sheets were then preparedof the cellulosic product obtained from a fiber-distilled water slurry,and using :a British sheet machine. These hand sheets were then pressedbetween blotters and air dried 18 hours, at 50% relative humidity, andat temperatures of 73 F. The hand sheets were conditioned in air at thesame relative humidity and temperature, for 24 hours before surfaceresistivity was determined. Surface resistivity was measured using a.Heathkit vacuum tube volt-ohmmeter, having a probe including twoparallel plates placed the same distance apart as the contacting lengthof the plates.

The following table summarizes the surface resistivities found for thesheets produced, and also shows the percent by weight of combinedphosphorus present in the cellulose ammonium phosphate making up thesheets.

TABLE I Curing conditions Phosphorus Con- Surface resistivity tent,percent by at 50% relative Run Time (min.) weight humidity, ohms persquareXlO a 3 1, 000 b 3. 72 100 c 11. 68 19 d 17. 49 5 Paper sheetsprepared as in Example I may be coated with a dielectric or insulatingfilm, with the production of copy sheets for electrostatic printingprocesses, as illustrated in the following example.

Example II A cellulose ammonium phosphate was prepared from sulfite pulpas in Example I. A relatively short curing time was employed (somewhatless than 3 minutes), and paper sheets prepared from the derivativeusing the procedure of Example I exhibited a surface resistivity of 1.5X 10 ohms per square at 50% relativehumidity. The paper sheets weresupercalendered 10 times at 4,000 p.s.i., and a dielectric coatingcomprising a styrene-butadiene polymer (about 80% styrene) was appliedover one face of the paper sheets using a spread of approximately 5pounds of polymer per 3,000 square feet of paper. The polymer, whenapplied, was dispersed in a 50% toluene, 50% methylethyl ketone solvent,as a 20% solution. Air drying of the sheets yielded electrostaticprinting papers or copy sheets, with essentially the same resistivitymeasured over the uncoated faces of the sheets.

Such papers were electrographically printed, using a Printapix cathoderay print out system, whereby imagedefining electrostatic charges weredeposited on the dielectric films forming one set of sides of the copysheets. The copy sheets were then developed, by dusting the surfaces ofthe films in the sheets with a developer substance, comprising a tonerhaving an electrostatic charge opposite to that of the charge on thesheets, and then permanently setting the toner on the surfaces of thecopy sheets by applying heat. Printed images were formed which exhibitedgood line resolution and had a minimum of background.

All of the paper sheets prepared as in Example I, when processed to formprinting paper, and printed as above described, exhibited similar goo-dline resolution and clarity.

The lphosphorylated cellulosic derivative prepared in Example Icomprises a monoammonium phosphate, and such a derivative yields paperof high brightness and good density, the derivative being the onepreferred for the manufacture of electrostatic printing papers ascontemplated herein. Other ionic forms of the cellulosic derivative maybe employed in manufacturing electrically conductive paper, andelectrostatic copy sheets from such paper. The following exampleillustrates the influence of different ionic groups on the electricalconductivity of paper made from a phosphate derivative of cellulose.

Example HI A cellulose ammonium phosphate was prepared and made intopaper sheets as in Example I. By determining the percent by weight ofcombined phosphorus in the derivative, based on the dry weight of theoriginally used cellulose, the degree of substitution of ionic groupsfor hydroxyl groups in the anhydroglucose unit forming the cellulosepolymer was determined. With each of such units containing threehydroxyl groups that may be phosphorylated, the maximum degree ofsubstitution (expressed as D.S.) to be expected is 3. The derivativeprepared, in its ammonium ion form, was used in the making of papersheets as in Example I. The derivative was also treated to change theion form of the derivative to one containing hydrogen, copper andaluminum ions, respectively, by dispersing quantities of the derivativein aqueous solutions of 10% hydrochloric acid, 10% cupric acetate, and10% aluminum sulfate, respectively, to effect a replacement of theammonium ion in the derivative with hydrogen, copper and aluminum ions.The cellulosic material resulting, having different ionic forms, wasthen washed and made into paper sheets as described in Example I. Thefollowing table illustrates for each of the ionic forms prepared thedegree of substitution calculated therefor, and the surface resistivityobtained in sheets prepared from the derivative in each of its forms.

Example IV The anhydrolglucose units that form the cellulose polymereach contain a primary hydroxyl group, which may be oxidized to yield acarboxyl group. Such oxidized cellulose is commercially available, andconstitutes another form of ionic fibrous cellulosic derivative that maybe employed in making the copy sheets of this invention. Such oxidizedcellulose, in an ionic form containing sodium and hydrogen ions,respectively, when used in preparing paper sheets as in Example I,yielded electrically conductive sheets as indicated in Table III below.In the case of oxidized cellulose, the maximum degree of substitution tobe expected is 1, for the reason that each anhydroglucose unit incellulose has but one primary hydroxyl group susceptible to oxidation.

TABLE III Surface resistivity, ohms per Derivative Ionic form D.S.square at 59% relative humidity Oxidized cellulose Na+ 0.67 140 Do H+0.67 125 The following example illustrates an anionic cellulosicderivative employable to make electrically conductive paper, andelectrostatic copy sheets with such paper forming the base sheettherein.

Example V An aminoethyl cellulose 'was prepared from cellulose, such assulfite pulp, and ethyleneimine, according to the method of Soifer andCarpenter Textile Research Journal, volume 24, page 847 (1954). Withaminoethyl cellulose, the hydrogen atoms of the three hydroxyl groups inthe glucose residue units in cellulose are replaced by ethyleneimine andthe maximum degree of substitution to be expected, therefore, is 3.Aminoethyl cellulose, due to its amino group, has a capability offorming amine salts which, in the presence of water, undergoesdisassociation with the liberation of anions, as for example, thehydroxyl ion. A derivative so prepared having a hydroxyl ionic form, anda degree of substitution of 2.5, was made into hand sheets as describedin Example I. These hand sheets had a surface resistivity at 50%relative humidity of 20 10 ohms per square.

Electrostatic printing papers or copy sheets may be prepared from thepaper sheets made in accordance with Examples III, IV and V by coatingfaces of the paper sheets with an insulating or dielectric film asdescribed in Example II. Paper sheets prepared from cellulose phosphatein an aluminum ion form tended to exhibit undesirable fiufliness, butafter coating with an insulating film, all sheets were usable in anelectrostatic printing process and had requisite electricalconductivity.

As discussed above, cellulose ammonium phosphate, when prepared tocontain a high percentage of combined phosphorus, produces paper sheetsthat, when made into copy sheets, may be emloyed to produce prints atrelatively low humidity conditions with good prints obtained. Inobtaining a relatively high percentage of combined phosphorus in aphosphorylated cellulose product, as contemplated by this invention, theamount of water present in the solution in which the cellulose isinitially immersed is limited to not more than about by Weight of thesolution. The solution itself comprises a weak organic nitrogenous base(and exemplary of these are such materials as urea and thiourea whichdonate the ammonium ions, such weak base resulting in a pH in thesolution within the range of about 5 to 8 whereby degradation of thecellulose is inhibited), and a phosphoric compound such asorthophosphoric acid. The solution itself should have a melting pointwhich is below about 150 C., preferably below about 125 C., wherebyduring the immersion step, temperatures may be maintained at a lowenough level to inhibit degradation of the cellulosic material. In 'atreating solution comprising urea and orthophosphoric acid, theproportions of acid in the treating solution ranges from to 80% byweight of the solution, with the proportion of urea comprisingessentially the remainder, and thus also ranging from 20% to 80%. Withthe preparation done as outlined above, a phosphorylated cellulosicproduct may be obtained containing from 15% to 20% combined phosphorus,and such a product is usable in making electrostatic printing copysheets where humidity conditions as low as about 15% relative humidity.

Example VI An ionic fibrous cellulosic derivative was prepared as inExample I, and subjected to a 15 minute curing time, to produce aderivative containing 17.49% combined phosphorus. The derivative wasrnade into paper sheets as described in Example I, and these sheetsconditioned at the relative humidities indicated in the table below (attemperatures ranging from 72 F. to 77 F.) for 24 hours. The sheets werethen tested for surface resistivity, with results obtained as summarizedin Table IV.

TAB LE IV Surface restivity, ohms per squareXlOt Run 50% relative 20%relative 15% relative humidity humidity humidity Example VII A celluloseammonium phosphate was Prepared from cellulose, as in Example I, andsubjected to a curing time of approximately 10 minutes at 180 C. Thisyielded a derivative which, when made into a paper sheet, evidenced asurface resistivity at 50% relative humidity of 22x10 ohms per square.Oxidized cellulose of the type described in Example IV, had a surfaceresistivity of 140x10 ohms per square, at 50% relative humidity. Thefollowing table sets forth the surface resistivities of blends of thesederivatives with ordinary sulfite pulp.

TAB LE V Surface Amount of resistivity at derivitive 50% relative inblend, humidity, percent ohms per squareXlO Run Derivative a Oxidizedcellulose b Cellglose ammonium phosphate. 0. o

Papers prepared from these blends were coated with a dielectric materialas set forth in Example II, and electrostatic prints made with the copysheets so produced exhibited good line resolution.

While an air convection oven may be used in curing the ionic fibrousderivatives, in some instances infrared heating may be preferable, asthis enables relatively high temperatures to be used without the dangerof surface scorching.

Example VIII Heavy blotter-type hand sheets prepared from bleachedsulfite pulp were immersed in an esterifying bath of the compositionindicated in Example I, maintained at a temperature of C. to C., for aperiod of about 5 minutes. The sheets were then removed from the bath,and pressed to retain about 300% by weight of the esterifying solution.The sheets were then cured in an infrared oven, macerated, and washeduntil the washings gave no test for phosphate. The following tableindicates the surface resistivities obtained for the various sheets,with such sheets cured at the temperatures and for the lengths of timeindicated.

TABLE VI Curing conditions Surface resistivity at 50% relative Runhumidity, ohms Temperature, C. Time, min. per squareXlO All of suchsheets, when prepared into copy sheets for electrostatic printing,produced good electrostatic prints. If desired, the impregnation andcuring steps may be combined, as illustrated by the following example.

Example IX Bleached sulfite pulp was thoroughly immersed in aphosphorylating mixture of the type described in Example I, with suchmixture in different runs maintained at the temperatures set forth inthe following table, and with the immersion in different runs continuedfor periods of time as indicated in the table. The pulp in the variousruns was then removed from the treating solution, washed and made intopaper sheets, and surface resistivities then determined.

TABLE VII Curing Conditions Surface Phosphorus resistivity at Runcontent, 50% relative Tempera- Time, percent by humidity, ture, 0. min.Weight ohms per squareXlO It will be apparent from the above that thereis contemplated by this invention a novel form of electrostatic printingproduct, where the support for the dielectric film in the productcomprises a web which is electrically conductive by reason of ionicdisassociation of the fibers making up the structure of the web. Theconductance of the web need not be the result of properties of anyadditive added to the pulp which forms the web. An additive may, onoccasion, be mixed with the pulp to produce certain characteristics ifdesired, however, with the invention the required conductivity in thebase web is produceable without employing such additives. As aconsequence, manufacturing procedures may be simplified, and trulyuniform electrical conductivity in the base web of an electrostaticprinting product is obtainable. This latter characteristic is believedto be a factor in making possible the use of papers in copy sheets withhigher resistivities than normally thought usable if satisfactory printsare to be obtained.

Various types of cellulosic derivatives may be employed to provide thefibrous structure in the web, including cationic and anionic forms. Websproduced from different types of derivatives are subject to somevariation, and in a particular manufacture the type of derivative whichis used is dictated by the characteristics desired in the final product.

In producing cellulose ammonium phosphates, a preferred form ofderivative, various procedures are employable. The curing times employedpreferably lie within the range of /2 to 20 minutes, and temperaturesare used ranging between 150 C. to 240 C. As indicated in the variousexamples, the length of curing time, and the curing temperatureselected, will affect the amount of phosphorylation which takes place.By proper selection of reaction conditions, a derivative containing ahigh degree of phosphorylation is produceable suitable for making copysheets for electrostatic printing usable at low relative humidities.

Although it is preferred to employ the ionic fibrous cellulosicderivatives as part of the stock mixture from which cellulosic webs areformed, these fibrous derivatives may be applied to a web made fromunmodified cellulose as a coating or impregnating solution or mixture ina suitable solvent or diluent such as water to render the web moreelectrically conductive. Such a coating or impregnating material may beapplied either to the side of the web upon which the dielectric film issubsequently applied, or to the side opposite that to which thedielectric film is subsequently applied, or to both sides of the webthereby to form at least a portion of the fibrous structure of the web.Similarly, such coating or impregnating material may be applied tocellulosic webs made at least in part of ionic fibrous cellulosicderivatives to impart additional electrical conductivity thereto.

While various examples have been included herein for the purpose offully illustrating the invention, it should be obvious that theinvention is capable of substantial variation. It is desired to coverall modifications and variations as would be apparent to one skilled inthe art, and that come within the scope of the appended claims. It isclaimed and desired to secure by Letters Patent: 1. In the manufactureof a printing paper, the process comprising preparing from fibrouscellulosic material an ionic fibrous cellulosic derivative whichdisassociates in the presence of water with the helease of charged ions,said derivative being the product formed by substituting ionizablegroups in the reatcive groups of the anhydroglucose units that formcellulose, preparing a web from the cellulosic derivative with suchderivative forming the fibrous structure of the web, and applying adielectric film over at least one face of said web with the film thenhaving an electrically conductive backing by reason of the ionic natureof the fibrous structure forming the web.

2. In the manufacture of an electrostatic printing product, the processcomprising collecting a mass of fibrous cellulosic material, immersingsuch mass in a phosphoric solution comprising phosphoric esterifyingmedium and a weak organic nitrogenous base, with such solution having amelting point below about C. and containing not more than about 15% byweight water, recovering the mass from such solution with the massimpregnated with solution, curing the impregnated mass by heating thesame at temperatures ranging from about C. to 240 C. for a period oftime ranging from /2 to 20 minutes, washing the resulting mass toproduce an ionic fibrous cellulosic derivative, preparing a cellulosicweb from the cellulosic derivative with the derivative forming thefibrous structure of the web, and applying a dielectric film over oneface of said web. 3. In the manufacture of an electrostatic printingproduct, the process comprising collecting a mass of fibers cellulosicmaterial, impregnating the mass with a liquid phosphoric esterifyingmedium, curing the impregnated mass to form an ionic fibrous cellulosicderivative of the cellulosic material through heating the impregnatedmass at elevated temperatures, said derivative being the product formedby substituting ionizable phosphate groups in the reactive groups of theanhydroglucose units that form cellulose, Washing the cellulosicderiavtive, preparing a cellulosic web from the cellulosic derivativewith the deriavtive forming the fibrous structure of the web, andapplying a dielectric film over one face of said web. 4. The process ofclaim 3, wherein the impregnation and 1 1 curing steps are carried onconjointly by immersing the mass in a heated liquid esterifying medium.

5. An electrostatic printing paper comprising a base Web having afibrous fabric composed at least in part of an ionic fibrous cellulosicderivative, said deriavtive being the product formed by substitutingionizable groups in the reactive groups of the anhydroglucose units thatform cellulose,

the cellulosic deriavtive making up the base web operating by the ionicnature thereof to impart electrical conductivity to the base web and theamount of such deriavtive in said web being sufficient to produce aresistivity of not more than 1.5 ohms per square at a relative humidityof 50%, and

a dielectric film joined to and extending over at least one face of thebase web.

6. An electrostatic printing paper comprising a cellulosic base sheethaving a fibrous structure composed of an ionic fibrous cellulosicderivative, said deriavtive being the product formed by substitutingionizable phosphate groups in the reactive groups of the anhydroglucoseunits that form cellulose,

said cellulosic derivative in said base sheet containing from 15% to byweight phosphorous, combined in the cellulosic derivative asphosphorylated cellulose, and

a dielectric film joined to the base sheet extending over at least oneface of the sheet.

7. An electrostatic printing paper comprising a cellulosic web whichincludes an ionic fibrous cellulosic derivative, said derivative beingthe product formed by substituting ionizable groups in the reactivegroups of the anhydroglucose units that form cellulose, said web havinga layer of dielectric material extending over at least one face thereof,and the web providing a conductive backing for the layer of dielectricmaterial by reason of the inclusion of the cellulosic derivative.

8. In electrostatic printing, wherein an image-defining electrostaticcharge is laid down upon one face of a dielectric film, the method ofdissipating electrical charges on the back ofsuch film by conductingsuch charges through a fibrous cellulosic derivative which forms a webprovided over the back of such film, such web having electricallyconductive properties through ionic dissassociation taking place in thecellulosic derivative, said derivative being the product formed bysubstituting ionizable groups in the reactive groups of theanhydroglucose units that form cellulose.

9. An electrostatic printing paper comprising a cellulosic web whichincludes an ionic fibrous cellulosic derivative selected from the groupconsisting of phosphorylated cellulose, oxidized cellulose, sulfoethylcellulose, carboxymethyl cellulose, and aminoethyl cellulose, and alayer of dielectric material extending over at least one face of saidweb, the derivative operating by the ionic nature thereof to impartelectrical conductivity to the web, the amount of such derivative in theweb being sufficient to produce a resistivity of not more than 1.5 1Oohms per square at a relative humidity of References Cited UNITED STATESPATENTS 2,357,962 9/1944 Leemann 162138 2,482,755 9/1949 Ford et a1.8116.2 2,987,433 6/1961 Ward 162157 3,161,505 12/1964 Tomanek 961.5

FOREIGN PATENTS 1,057,440 5/ 1959 Germany.

OTHER REFERENCES Phosphorylation of Cotton With Inorganic Phos phates,American Dyestuff Reporter, vol. 53, No. 10, pp. 23-25, May 11, 1964(copy in 8120).

S. LEON BASHORE, Primary Examiner.

US. Cl. X.R.

